1. Field of the Invention
The invention relates generally to the production of consolidated cellulosic products and, more specifically, to the production of a consolidated cellulosic product using a steam injection pressing operation.
2. Brief Description of Related Technology
Synthetic resin adhesives, such as phenol-based resins, are widely used as binders in the manufacture of composite articles, such as consolidated cellulosic products (e.g., waferboard, chipboard, oriented strandboard, or fiberboard). Such composites may be formed by various processes and may be formed in a variety of desired shapes and sizes depending on the intended end use of the composites. Generally, however, consolidated cellulosic products are formed by combining a phenolic resin, such as a phenol-formaldehyde resin, with filler material, such as cellulosic fibers or cellulosic particles, and then bonding the filler material together in the presence of heat and pressure. Various processes are described in U.S. Pat. Nos. 5,367,040 and 5,637,658 and co-pending, commonly-assigned U.S. application Ser. No. 08/888,878 (filed Jul. 7, 1997), the disclosures of which are hereby incorporated herein by reference.
A principal process of making a consolidated cellulosic product is a xe2x80x9cdryxe2x80x9d process. In a dry process, filler material, such as cellulosic fibers, is generally conveyed in a gaseous stream or by mechanical means in a felting step. For example, the fibers supplied from a fiberizing apparatus (e.g., a pressurized refiner) may be coated with a thermosetting synthetic resin, such as a phenol-formaldehyde resin, in a blowline blending procedure, wherein the resin is blended with the fiber with the aid of air turbulence. Thereafter, the resin-coated fibers from the blowline can be randomly formed into a mat by air blowing the fibers onto a support member. Optionally, the fibers, either before or after formation of the mat, can be subjected to pre-press drying, for example in a tube-like dryer. The formed mat, typically having a moisture content of less than about 30 wt. % and preferably less than about 10 wt. %, is then pressed under heat and pressure to cure the thermosetting resin and to compress the mat into an integral consolidated structure. A xe2x80x9cwet-dryxe2x80x9d process uses water to convey the cellulosic material and binder, prior to one or more water removal, e.g. drying, steps.
Steam injection pressing is a consolidation step that can be used, for example, under certain circumstances in dry and wet-dry process production of consolidated cellulosic composites. In steam injection pressing, steam is injected through perforated heating press platens, into, through, and then out of a mat that includes the synthetic resin and the filler material. The steam condenses on surfaces of the filler and heats the mat. The heat transferred by the steam to the mat as well as the heat transferred from the press platens to the mat cause the resin to cure. When compared with conventional pressing operations, steam injection pressing may, under certain circumstances, provide a variety of advantages, such as, for example, shorter press time, a more rapid and satisfactory cure of thicker panels, and products having more uniform densities.
Steam injection pressing of mats containing conventional phenolic resins, however, often results in a final composite product exhibiting undesirable characteristics, such as poor bond formation and/or starved glue lines. In order to ensure good bond formation, it is desirable to have a uniform dispersion of the phenolic resin throughout the mat. However, because phenolic resins are water soluble, when mats containing such resins are steam pressed, the steam may condense and may solubilize the resin. The solubilized resin may undesirably migrate to regions of the mat such that the resin is no longer uniformly dispersed, resulting in a product that may have resin-starved regions and poor bond formation. Poor bond formation also is attributable to the known phenomena of pre-cure (i.e., where the resin cures before the mat has hardened to an integral consolidated structure) and moisture retardation (i.e., where water present in the core or interior of the mat prevents the mat temperature from exceeding the evaporation temperature of water, 100xc2x0 C., thereby retarding the resin cure). Starved glue lines caused by excess penetration of the resin can occur near the surface of the formed product where resin has undesirably been washed from the surface of the mat and has migrated to edges of the mat or to the mat core. The absence of resin near the product""s surfacexe2x80x94hence, the presence of starved glue linesxe2x80x94causes flaking of the product.
In view of the foregoing, it would be desirable to provide a method of making a cellulosic product that overcomes the problems described above. More particularly, it would be desirable to provide a method of making a consolidated cellulosic products, using a conventional phenolic resin and a steam injection pressing operation, that retains the advantages and overcomes the disadvantages of prior methods of making cellulosic composites.
It is an object of the invention to overcome one or more of the problems described above.
Accordingly, the invention provides a cellulosic composite article and methods of making the same. Generally, the method includes the steps of combining a phenolic resin with cellulosic material to form a mixture, forming a mat from the mixture, and consolidating the mat under heat, steam, and pressure in a pressing apparatus to form the article. The method further includes the step of providing a catalyzing agent, such as aluminum chloride, to the mixture prior to the mat formation step to acid hydrolyze the polymeric molecules comprising the cellulosic material. As an alternative to these steps, the method may include the steps of combining a catalyzing agent, such as aluminum chloride, with a cellulosic material to form a mixture, forming a mat from the mixture, and consolidating the mat under heat, steam, and pressure in a pressing apparatus to form the article. In this alternative method, the method further includes the step of providing a phenolic resin to the mixture prior to the consolidation step.